This invention relates generally to the field of injection molding and more particularly to an improved nozzle assembly for use in injection molding operations that involve molding of thin walled parts using continuous high volume molding at high pressure and high speed.
Injection molding equipment has long made use of nozzles for injecting the molten plastic into the mold cavities. Thin walled parts have been commonly manufactured by this type of injection molding process. Thin walled parts such as plastic cups and other articles are usually molded using continuous high volume molding techniques carried out at high pressure and high speed. There is no shut off mechanism for the nozzle in a continuous molding operation of this type. Cycles are carried out successively at high speed and rapid injection of molten plastic with the nozzle continuously open to flow. High pressures are also required in this type of molding process. By way of example, flow through the injection nozzle during each cycle occurs for approximately 0.2 second. Pressures of between 20,000 psi and 30,000 psi are typically involved. This is in contrast to molding operations for larger parts that take place much more slowly and at much lower pressures.
It has been the normal practice in the past for the nozzle assembly to be formed by a housing and a probe or nozzle head which is screwed into the housing. Although this type of arrangement has been widely used, it has not been wholly without problems in applications involving continuous high volume molding operations.
Perhaps the most serious problem has been the tendency for the nozzle to come loose from the housing. The repeated application of high pressures in rapid succession during the molding process can inadvertently work the threads of the nozzle loose from the threads of the housing. Eventually, the nozzle completely detaches from the housing. When this occurs, the mold must be shut down, and the parts that are the source of the problem must be located, cleaned and either reassembled or replaced before the molding equipment can resume operation. This can result in considerable down time for the molding machinery and can cause large financial problems due to lost production. Modem molding equipment for thin walled parts often makes use of molds that provide a large number of mold cavities, so the production losses can be substantial each time even one of the nozzles works loose from its housing.
The high temperatures that are involved also create problems. If the nozzle head is rigidly secured to the nozzle housing, the nozzle or other components can crack due to the thermal expansion that occurs under high temperature operation. The high pressures and fast cycle turns involved in the high volume continuous molding process exacerbate this problem. If a part cracks or is otherwise damaged due to thermal problems, the downtime of the machinery creates all of the difficulties discussed previously. Accordingly, it is of considerable importance to recognize and make provision for thermal expansion effects.
It is the principal object of the present invention to provide a nozzle assembly for injection molding which functions more reliably than existing nozzles and which is not subject to detachment of the parts that can cause undue downtime of the molding equipment.
Another object of the invention is to provide a nozzle assembly of the character described which is not subject to cracking or other damage due to the effects of thermal expansion.
It is also an object of the invention to provide an improved nozzle assembly of the character described which is simple and economical to manufacture, easy to use properly, and applicable to existing equipment.
The problems which have plagued existing nozzle assemblies are eliminated or at least minimized by the nozzle assembly of the present invention. In accordance with one embodiment of the invention, a three-piece nozzle assembly includes a nozzle housing, a nozzle, and a jam nut. The housing has a barrel which presents an internally threaded bore in one of its ends. The nozzle has an externally threaded shank which is screwed into the bore and tightened against an internal shoulder at the end of the bore. The jam nut is threaded onto the nozzle shank and is tightened against the end of the housing. This securely locks the nozzle to the housing so that the pressure applied during operation will not cause the nozzle to screw out of the housing as occurs with existing devices.
In accordance with another embodiment of the invention, which is preferred for most applications, the nozzle is threaded into the bore of the housing and has a projecting flange which abuts an internal shoulder in the bore when the nozzle is fully installed. A threaded sleeve or collar is applied around the nozzle and is threaded into an enlarged portion of the bore and tightened against the flange. The sleeve prevents the nozzle from working loose during operation of the molding equipment and is threaded to the housing rather than to the nozzle. This minimizes the area of the sleeve in contact with the nozzle to minimize heat conduction to the sleeve. A gap is presented between the nozzle and sleeve for a major part of the sleeve length to further reduce heat transfer. Preferably, a small gap is present between the end of the nozzle and the shoulder it abuts, and also between the flange and the sleeve so that thermal expansion is accommodated without causing the parts to crack or deform unduly at high temperatures.